Frustrating Skies and Collimation Questions

I drove 75 miles tonight to a wonderful dark site. Clouds were moving in while driving and the chances of good seeing were gonna be less than 50/50. By 10:00 I gave up with about mag 2 skies. I can get that in my backyard! All geared up and nothing to see. Bur prior to departure, I worked on collimating my coulder 13.1 f4.5 dob. I don't know which is more frustrating, overcast skies or collimating a fast scope. I'm learning about collimation. Today I fiddled with the secondary. The previous owner has placed a very small dor on I assume the optical center, althouth it may be the mechnical center. I guess I could measure to find out. I could not get the dot in the crosshairs of my cheshire. I could get it on the horizontal line of the crosshair and just a smidge to the lest, but not on center. I then centered the donut on the primary and I must say, this is the best it has ever looked through the cheshire. Practice, practice, practice! The secondary is attached to a thick steel bar, original to the scope. Tonight was the first time I say two distinct spikes when viewing Jupiter (about the only object visible). I assume the secondary is lined up properly now because I've read that 4 spikes are seen with a 4-vane spider. Is this a correct assumption? Also the airy discs were much more prominent and better centered. I have a laser collimator on order. Should better results that with a cheshire? Any collimation gurus out there. Thanks in advance.

David Roberts

David,

I'm not a collimation guru, but I've been practicing both with a cheshire and a laser collimator on my 18" dob. The results you're getting on your star tests indicate you're on the right track. Regarding laser collimators, don't assume you can get better results than with a cheshire. The view through a cheshire gives you a picture of all the elements in the optical path. With experience, you can determine exactly what's out of line. With a laser, if the return dot doesn't return to the source, you can't tell why. It's all too easy to assume it's one component or another, and to start fiddling with primary collimating adjusting bolts to get the return dot back in the source. What results is a systematic collimation error. I've found the cheshire to be a better tool to use just after setup. After using the cheshire, I use the laser to evaluate the collimation.

Good luck,
Howard Knytych

Dave –

An often overlooked but critical aspect of collimation is ensuring that the focuser is square to the optical path. If it is not square, then you will have great difficulty in getting the rest aligned. In fact, this is where you have to start, before you align anything else.

I struggled with collimation in my 13” f/4.5 dob literally for years until I discovered that my focuser was a good 5 degrees off laterally. Once I got it straightened out, the rest just fell into place.

This is where a laser collimator really shines (pun intended!). You can measure from the end of the tube to the projected dot to ensure that it is square longitudinally, and ensure that it hits the center line of a dowel or threaded rod inserted through the mounting hole of your exactly centered (side-to-side) secondary support to ensure squareness laterally.

After you’re sure that the focuser is square, insert the secondary and get it positioned correctly under the focuser, then adjust it’s rotation and tilt, and then finish up by adjusting the primary. You can do these with a Cheshire or a laser, or both.

Best Regards,
Doug